1. Technical Field
The present invention relates to a metal tube for reinforcing car body, which provides an excellent crash capacity in case of a car body collision, along with a reduction in the weight of the car body, and the present invention also relates to a car body reinforcement produced therewith.
2. Description of the Related Art
In recent years, requirements on the safety for a car body are increasing in the automobile industry, and the technology for protecting occupants in the case of a collision has been developed. In conjunction with the facts, for improving performance of protecting occupants, the structure of car body components inclusive of doors in a vehicle was reconsidered and tried to employ reinforcements for efficiently protecting occupants.
FIG. 1 is a drawing showing the structure of a car body in which the reinforcements are used, as example, as door impact bars for a door in an automobile. The door 1 is formed in such a way that a window frame 3 is disposed onto the upper part of a door inner 2 in one-piece structure, so that it is constituted by jointing a door outer (not shown) to the door inner 2 on the car outside. Moreover, the door 1 is connected to the car body 4 on the front side thereof (left side in the drawing) by means of upper and lower hinges 5a and 5b, and on the rear side (right side in the drawing), the door is further equipped with a door lock 6 at the middle level of the door inner 2. Such a structural arrangement allows the door to be opened/closed freely and to be secured in the close position by the door lock 6.
Each of the above-described door impact bars (reinforcement) 7 has brackets 8 for connecting the impact bar to the car body at both ends thereof. In such a constitution of the reinforcement shown in FIG. 1, the bracket 8 is connected either to the hinge 5 or to the door lock 6. Accordingly, a door impact bar 7a is interposed between the portion onto which the upper hinge 5a is mounted and the portion onto which the door lock 6 is mounted, and the other impact bar 7b is interposed between the portion onto which the lower hinge 5b is mounted and the portion onto which the door lock 6 is mounted.
The above-described reinforcement shown in FIG. 1 has a structure in which brackets are used to connect tube ends to the car body, thereby enabling the brackets to be connected to the car body. In another embodiment, a so-called bracket-less structure is employed, wherein no bracket are disposed at both ends of the reinforcement, and therefore both ends of the reinforcement are directly connected to the car body.
Recently, a requirement for reducing the weight of a car body in an automobile is increasing in view of requirements for decreasing the fuel consumption and/or the manufacturing cost. As a result, the promotion of the weight reduction is also strongly required in such a reinforcement for enhancing the safety. For such a requirement, several methods have been proposed for reducing the weight by using steel tubes and/or steel sheets having high strength in a conventional reinforcement.
For instance, Japanese Patent No. 2811226 has disclosed a steel tube for reinforcing car body having both a tensile strength of not less than 120 kgf/mm2 (1180 MPa) and an elongation not less than 10% for such a core element as a door impact bar, a bumper or the like, which is used as a steel tube for reinforcing the door to ensure the safety of a driver in the case of a side collision of an automobile. In accordance with the properties of the steel tube, the weight of the steel tube for reinforcing car body can be reduced and the energy of impact can be efficiently absorbed by selectively determining the shape of the steel tube as for given conditions of operation.
Furthermore, a method for using a pipe to reinforce a door has also been proposed, wherein the pipe is produced from a steel tube having a low strength or a steel sheet having a low strength, and then the tensile strength is enhanced by applying the quenching process thereto. For instance, such a method is disclosed in Japanese Patent Application Publication No. 04-280924, wherein a straight steel pipe is directly heated up by supplying an electric current thereinto, and then quenched by injecting cooling water to the steel pipe under a condition that a tensile force is applied to the steel pipe in order to avoid bending.
As described above, the car body reinforcement produced from a conventional steel tube provides to some extent an increase in the mechanical strength and a reduction in the weight, and it is prerequisite to use a straight tube in any case. However, it is found that such a conventional reinforcement produced by a straight tube provides a limitation in the weight reduction and in enhancement of collision property.
FIG. 2 shows diagrams showing the dependence of the load on the displacement of a striker in the three-point bending test (supported at both ends) for a steel tube to explain the impact characteristic. The impact characteristic is evaluated as for the following items: the load ramp gradient representing the characteristic for an applied load at the start of deformation; the maximum load representing the maximum value of the load; and the absorbed energy which can be expressed as a value obtained by integrating the load with respect to the displacement.
That is, regarding the crash capacity required for a car body reinforcement, it is necessary to increase the load ramp gradient, the maximum load and the absorbed energy when the reinforcement is deformed due to the collision from the outside, in particular to increase the load ramp gradient as for the load property at the start of deformation. In view of this fact, FIG. 2A shows the behavior of deformation in a steel tube having an excellent crash capacity, whereas FIG. 2B shows the behavior of deformation having an inferior crash capacity, compared with that in FIG. 2A.
Although both the maximum load and absorbed energy can be ensured to some extent by employing a material having a high strength in a reinforcement prepared from the conventional straight tube, it is impossible to enhance the load ramp gradient, and to provide an increase in the applied load at the start of deformation, and therefore there is a limitation in the required crash capacity.
Japanese Patent Application Publication No. 04-280924 has provided a description regarding the manufacture of a curved tube as an exception of the usage of a straight tube, which is conventionally used for the reinforcement. Actually, a method for manufacturing a reinforcement is also disclosed, wherein a straight steel pipe is heated by directly supplying an electric current thereinto and then quenched by injecting cooling water thereto, as well as by pressing it onto a mold so as to obtain a desired shape for a curved steel pipe.
However, the door guard beam described in Japanese Patent Application Publication No. 04-280924 is invented by paying attention to the fact that each door of automobiles in most cases has a specific roundness in association with the design of an automobile, and the pipe has only to be curved by fitting it to the roundness of the door. Accordingly, the above specification aims to provide a well-designed automobile, so that the door guard beam described in Japanese Patent Application Publication No. 04-280924 pertains merely to the reinforcement produced by a straight tube.
Moreover, in the door guard beam described in Japanese Patent Application Publication No. 04-280924, it is not aimed either to increase the applied load at the start of deformation, or to increase the maximum load, or further to increase the absorbed energy by using a curved pipe, as will be described below.
In addition, as for the door guard beam disclosed in Japanese Patent Application Publication No. 04-280924, a method for producing such a door guard beam is restricted to the process where a straight pipe is directly heated up by supplying an electric current and is cooled by injecting cooling water under the condition that it is pressed onto a bending mold. In such a method, it is impossible to uniformly cool and quench the tube over the whole length, thereby causing a possible unevenness to generate in the quenching.
As a result, in the door guard beam produced with the bending process in Japanese Patent Application Publication No. 04-280924, there is a possibility that an instability in the quality results from the generation of the unevenness in the quenching, and further the precision in the size and/or the shape cannot be obtained due to the uneven deformation, thereby making it impossible to use it as a finished product.